The Portland daily press., 1887-08-31
AIvan cIark's Care in Making Lens


On table in Alyan Clark's manufactory
at Cambridge was the finished Pulkoya lens
which weighed 450 pounds, and consisted of
two lenses, each 30 inches in diameter saws
Otto Michels in the Sun. Generally these
lenses are made to accurately fit, and are
Joined together with Canada balsam. but in
such large glasses as the Pulkova and the
Lick, they are fitted in metal frame with
an adjustment so that they can be made to
approach each other, or otherwise II sin :
gle lens were used, the object inspected
would be fringed with various artificial col
ors and other defects due to spherical aber-
ration, but by the simple artifice of using
two kinds of glass these defect. are cured to
great extent and a nearly perfect image is
secured.
When the evening was sufficiently adj
yanced the great Pulkova glass was placed
in its temporary fitting in the garden. There
was no moon and the darkness was intense
The glass was brought out on a four-wheel
hand truck, and lifted into the tube by five
men, and fixed by revolving it in the screw
lifting. The tube was 40 feet long, and
weighed with the attending fittings about
seven tons. Two piles Of brickwork sup
ported the whole; There was no closkwork
INV SnoiI, anu VIC toughest apparatus was
employed. The telescope was raised and
moved by iguide rope, the motion of an
equatorial movement being initiated by using
a common windlass. AS the motion of the
earth caused the object to pass across the
held Of the telescope, the observer gave the
order follow" when a slight turn of the
windlass kept the object in view. Such
were the rough appliances used to test this
100000 lens.
The planets had all set, and l had to be
satisfied with a view of a fixed star, which is
an excellent object for testing the Optical
properties Of a lens, but very uninteresting
otherwise, as the largest telescope can make
little impression on fixed star no disk can
be seen, merely speck of light. The star
selected was small one, and barely visible
as pale, minute object. On looking at it
with this magnificent instrument its wonder
full light gathering powers were at once evil
dent, fer the star shone with the luster and
brilliancy of an electric light. 1t was an OBJ
jet which brought out all the imperfections
of the glass, and to the eyes of Mr.. Clark and
his sons many were evident, and, it was
said, two months work was necessary to
correct them. During the trial the lens was
lowered and five men revolyed the glass in
It's lifting. On its being placed in its position
again one OF the sons was about to make
as .9 test when the old man shouted:
wan. boys; let her COOL"
l was curious to know what this could
nlean, ana Alvan Clark explained that the
ttpHarsectl9n was SO delicate that the heat from
one hanas OF the five men holding the metal
case Of the objective would change the COR
reetlon, so 1t had to cool"
On the front or crown glass lens was n
se sfrata a[ TX aW due to the appng over Of
ionnd u 70 be l inches long. bv one-eightb
OF an inch in diameter, almost in the csn uer
OF the lens, near the front surface This 1,,es
a very provoking circumstance in a $40 ooo
glass, SIS. Mr.: Clark had ordered another disc
to accept the damaged one, at least for a
time, as it seemed very doubtful when anothl
er glass might be expected to be made in
France. This flaw would not alter the def
milton of the lens, but would merely stop
fractional part Of light passing through the
lens,